In my prior U.S. Pat. No. 4,131,761, owned by the assignee of the present application, I have disclosed a method and a system of this type in which primary information is transmitted over a signal path in the form of a stream of symbols with three or more amplitude levels, i.e. two limiting (minimum and maximum) levels and one or more intermediate level. These symbols may be transmitted directly or via a carrier as amplitude-modulated pulses or, possibly, with intervening frequency modulation. Symbols with amplitudes on at least one limiting level are referred to as "modulable" since they can be used, in conformity with the teaching of my prior patent, for conveying the bits of a supplemental message word of a supervisory nature, for example; a bit of logical value "1" is characterized by an extension of the symbol amplitude beyond its normal limiting level to an extra-high or an extra-low level. These excursions beyond the normal amplitude range do not affect the recovery of the primary information from the symbol stream at a receiving end of the signal path since a threshold comparator used for this purpose will not distinguish between the maximum and the supermaximum (or the minimum and the subminimum) level.
Since modulable waveforms occur randomly in such a symbol stream it is necessary to correlate their demodulation for the recovery of the supplemental message at the receiving end with their supermodulation at the transmitting end. For this purpose it is convenient, as described in my prior patent, to organize the symbol stream at the transmitting end into a recurrent outgoing frame encompassing a fixed number of symbol periods, the number N of these symbol periods being large enough to insure that the probable number of modulable waveforms in each frame substantially exceeds the number k of bits in a binary word coding the supplemental information to be transmitted with each frame. The carriers for these bits are then the first k modulable waveforms occurring in a given outgoing frame; when the symbol stream arriving at the receiving end is similarly organized in a recurrent N-period incoming frame synchronized with the outgoing frame, these bits can be readily retrieved from the first k modulable waveforms of each incoming frame which may or may not be supermodulated.
The problem of synchronization is solved in the system of my prior patent by dividing each frame into a plurality of subframes and using the first subframe of each frame for the transmission of a multibit alignment word to the exclusion of supplemental message bits. The alignment word comprises an invariable first part with a characteristic bit combination and a variable second part which indicates the number of symbol periods or clock cycles elapsed between the beginning of the frame and the occurrence of the first modulable waveform therein; that information enables a decoder at the receiving end to determine the starting points of the remaining subframes for a correct decoding of their respective supplemental messages.
While the LDM system of my prior patent operates generally satisfactorily, the need for using an alignment word reduces the number of supplemental message bits that can be transmitted in each frame.